Optical sources that can produce several intensity-modulated signals at different frequencies simultaneously are becoming increasingly important for wavelength-division-multiplexed optical communications. Such sources usually consist of several lasers operating at different optical frequencies that are either multiplexed extra-cavity (e.g., FIG. 1a) or intra-cavity (e.g., FIG. 1b). The components for such multi-frequency sources are often integrated in a photonic circuit, and there is often an optical output amplifier (e.g., 101) through which all the signals pass. This optical amplifier is necessary to boost the power of the optical signals. However, the amplification of the optical signals causes carrier density changes in the optical amplifier. The carrier density changes in turn affect the optical amplifier gain, resulting in nonlinear operation which produces signal distortion and crosstalk between the signals.